Organisms continue to develop and change throughout life (Kinsley, 1994). Indeed, brain development is not confined to the period of life when initial growth and systems elaboration are occurring. In particular the adult female brain is modified, reorganized in some examples, in surprisingly rapid fashion as during the estrous cycle after exposure to gonadal steroids (McEwen & Woolley, 1994; Woolley et al., 1990; Woolley & McEwen, 1992, 1993). Little attention has been paid, however, to the remarkable neural plasticity inherent to reproduction itself, particularly endocrine-neuron relations that may be unique to pregnancy and the postpartum period. What might such changes mean for the female's behavior subsequent to the reproductive events themselves? Do there exist long-lasting and pervasive alterations in the female that accompany the events of pregnancy? Specific hormone-induced changes in the brain may, alone or in interaction with the sensory tumult provided by the newborns, facilitate the female's learning of the new behavioral repertoire required to care for them. We ask here if the hormones of pregnancy interact with postpartum pup exposure to produce an "enriched environment" and improve learning ability (at least temporarily and possibly permanently) in the mother. Further, if there are behavioral effects due to the hormones of pregnancy, are there changes in the neural substrate responsible for the alterations of learning, especially in hippocampus? Lastly, given observed and the above-hypothesized changes in hippocampal neuronal characteristics, would stress mitigation be among those variables subsequently influenced by the pregnancy hormones? The present application examines to what extent such reproductive experiences have ramifications for maternal survival behaviors such as the ability to negotiate an environment in order to find food. Further, we will examine the potential stress-mitigating effects of pregnancy hormone exposure in the hippocampus. In the experiments to follow we will address the following Specific Aims and Experiments: SPECIFIC AIM 1. Because the female rat brain is exposed to high and substantial levels of ovarian hormones during pregnancy hormones associated with alterations of hippocampal neuronal morphology are learning and memory significantly altered? Experiment 1: Does reproductive experience enhance spatial and non- spatial learning? Experiment 2: Do multiple parity experiences result in enhanced learning ability? Experiment 3: What is the role of pup stimulation in enhancement of maze learning? SPECIFIC AIM 2. What are the hippocampal neuronal modifications following pregnancy and lactation? Experiment 4: Does parity alter hippocampal pyramidal and glial cells? SPECIFIC AIM 3. Does reproductive experience/parity mitigate stress responsiveness in the female? Experiment 5: Does the experience of parity provide protection from subsequent exposure to chronic stress challenges?